A multitude of wireless communications systems are in common use today. Mobile telephones, pagers and wireless-connected computing devices such as personal digital assistants (PDAs) and laptop computers provide portable communications at virtually any locality. Wireless local area networks (WLANs) and wireless personal area networks (WPANs) according to the Institute of Electrical and Electronic Engineers (IEEE) specifications 802.11 (WLAN) (including 802.11a, 802.11b/g, 802.11n, etc.), 802.15.1 (WPAN) and 802.15.4 (WPAN-LR) also provide wireless interconnection of computing devices and personal communications devices, as well as other devices such as home automation devices.
Within the above-listed networks and wireless networks in general, it is desirable to know the location of devices for operation of location-based services, mapping of network facilities, and security. The above-incorporated parent applications describe a method and system for synchronizing location finding measurements in a wireless local area network and a system for location finding within a wireless network. In both applications, the method and system use the time-difference-of-arrival (TDOA) of a transmitted signal received at multiple receivers within a wireless network in order to determine the position of the transmitting device via triangulation.
In the TDOA location finding system described above, synchronization of devices is essential so that accurate TDOA measurements are made between the network devices. Synchronization in the above-referenced parent application is achieved by continuously estimating the drift and difference between clocks of each of the location-finding units in a server or central processor.
Hard-wired synchronization schemes may distribute a clock from a central source, but distribution of a clock is a costly alternative to synchronization. Hard-wired schemes that pass synchronization error information over a wired network connection as well as wireless synchronization schemes are preferable, but rely on stability and accuracy of the local clock in each location receiver.
Synchronization of clocks by TOA measurement of wireless messages transmitted by units at known locations is valuable but can cause synchronization errors due to offsets. Those offsets are commonly caused by multipath and/or indirect path signals which lead to TOA measurement errors. When those offsets are constant over time they are hard to detect since they are interpreted by the system as initial offsets of the clocks. In some systems, it is possible to cancel this offset by an initial calibration but those offsets tend to change over time.
Therefore, it would be desirable to provide a method and system for continuously and accurately measuring and reducing synchronization offset errors within a TDOA-based location finding system that also introduces little or no synchronization overhead.